The Flea

by Harold Russell

The mouth-parts of fleas are differently constructed from those of all other insects. Around the orifice of the mouth are a number of appendages which form a complicated apparatus for piercing and sucking. Their construction and use cannot be described without employing some technical terms. When the names of the parts have been mastered, a diagram will make their relative positions clear. It may be necessary, first, to remind the reader who is not an entomologist that the real mouth of an insect is the entrance to the alimentary canal, and that the appendages of the mouth, which act like jaws for masticating or like tubes for sucking, are really modified limbs. In fleas the mouth is suctorial. But before sucking up the blood the flea must first pierce the skin of its host. The paired mouth-parts, then, are modified limbs which correspond with those appendages on the thorax of an insect which we call the three pairs of legs.

The primitive insect, of which fleas and all other insects are descendants, was, it is supposed, composed of a succession of segments each bearing a pair of jointed appendages. Insects of the present day never have more than six legs, but the foremost pairs of appendages have been bent round, reduced in size, and altered in shape so as to serve as mouth-parts.

Now the mouth-parts of the flea for which only technical names exist are the maxillæ and maxillary palpi, the labium and labial palpi, the mandibles and the labrum. The labrum is considered by some authorities to be the hypopharynx. It will be best to deal with each of these in turn and then to explain how they act in combination.

The maxillæ. These are a pair of horny or chitinous triangular plates one on either side of the flea’s face. They are placed some distance away from the orifice of the mouth and to the right and left of it. They do not serve for piercing or sucking, and appear to have no active function unless they serve to separate the hairs of the host and enable the flea to reach the bare skin. In the majority of bat-fleas (Ceratopsyllidæ) the maxillæ are dumb-bell-shaped but in all other fleas they are more or less triangular. From the fore part of each springs a palpus. Like other highly chitinised parts of a flea, the maxillæ are usually dark in colour.

The maxillary palpi. These are jointed hairy feelers which project forwards and were mistaken by the older naturalists for antennæ. They spring from the base of each of the maxillæ where these latter organs are joined to the head of the flea. The palpi are sense-organs as the number of sensitive hairs on their surface indicates. The maxillary palpi of fleas are always composed of four segments.

The labium and labial palpi. These form together what is called the rostrum of a flea. The labium is a single organ which projects beneath the aperture of the mouth. It may be described as the lower lip of the flea. At its end it divides into two comparatively long branches. These are the labial palpi. The actual piercing organs, which will be described below, are the mandibles and labrum. They are not so conspicuous as the rostrum which protects them.

When the piercing organs are at rest they are partly retracted. The external portion is encased in the tubular rostrum. The tube is formed by the two labial palpi which are situated at the apex of the short non-divided labium. The number of segments composing each labial palpus in fleas varies, so far as we know, from two to seventeen. In most fleas, however, the labial palpus consists of five segments. This appears to have been the original state of things in the ancestral flea; the palpus with more and the palpus with less segments being derived from the normal five-jointed one. The rostrum of a flea is not a piercing organ like that of a fly and a bug. The two labial palpi separate and lie flat, right and left, on the skin when the true piercing organ is driven into the host. The labial palpi therefore require to be flexible, and this is attained by increasing the number of segments or by reducing the amount of chitinisation or horniness. We shall find in the chigoes and their allies a rostrum which is pale, weak, soft and scarcely horny. Among other fleas where the rostrum is prolonged and strongly chitinised we shall find greater segmentation.

The small bristles at the extreme tip of the rostrum seem to be sensory organs. They are like those at the apex of the maxillary palpus. When a hungry flea is put on one’s arm, it appears to test the skin with these bristles before it ventures to make a puncture.

The mandibles. These are a pair of sharp lancets with serrated edges. They make the puncture and are interlocked with the labrum to form a sucking tube.